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+---
+title: "Quick Python: Check Submodule Load"
+date: 2022-12-01T21:12:03-05:00
+draft: true
+tags: ["Python"]
+math: false
+---
+
+Reading other people's code is a great way to learn. Recently, I was looking at the `numpy` repository and they had a hidden gem within their `setup.py`. In large scale repositories like `numpy`, we can have different dependencies that we rely upon via *git submodules*. The following function will check to see that these dependencies are loaded and in a "clean" state.
+
+```python
+def check_submodules():
+    """ verify that the submodules are checked out and clean
+        use `git submodule update --init`; on failure
+    """
+    if not os.path.exists('.git'):
+        return
+    with open('.gitmodules') as f:
+        for line in f:
+            if 'path' in line:
+                p = line.split('=')[-1].strip()
+                if not os.path.exists(p):
+                    raise ValueError('Submodule {} missing'.format(p))
+
+    proc = subprocess.Popen(['git', 'submodule', 'status'],
+                            stdout=subprocess.PIPE)
+    status, _ = proc.communicate()
+    status = status.decode("ascii", "replace")
+    for line in status.splitlines():
+        if line.startswith('-') or line.startswith('+'):
+            raise ValueError('Submodule not clean: {}'.format(line))
+```
+
+To get a better idea of what this piece of code is doing, let's trace through its execution. The `.gitmodules` file defines where each of the submodules are stored in the repository. Here's the file for `numpy`'s repository.
+
+```ini
+[submodule "doc/source/_static/scipy-mathjax"]
+	path = doc/source/_static/scipy-mathjax
+	url = https://github.com/scipy/scipy-mathjax.git
+[submodule "numpy/core/src/umath/svml"]
+	path = numpy/core/src/umath/svml
+	url = https://github.com/numpy/SVML.git
+```
+
+The function then checks if each of the `path` locations are defined in the filesystem.
+
+```python
+# Check #1
+if not os.path.exists("doc/source/_static/scipy-mathjax"):
+    raise ValueError('Submodule {} missing'.format("doc/source/_static/scipy-mathjax"))
+
+# Check #2
+if not os.path.exists("numpy/core/src/umath/svml"):
+    raise ValueError('Submodule {} missing'.format("numpy/core/src/umath/svml"))
+```
+
+Now it is possible for these submodules to be checked out, but be in some sort of unclean state. For example let's say that I edit `numpy/core/src/umath/svml/README.md` and commit that change. Then the `git submodule status` command will look like the following.
+
+```bash
+$ git submodule status
+36f4c898f2255e0c98eb6949cd67381552d5ffea ../doc/source/_static/scipy-mathjax (heads/master)
++1f2b5b3cae704ffa6210a638dfd084946da366bf core/src/umath/svml (heads/main-1-g1f2b5b3)
+```
+
+Notice the `+` behind the second submodule. According to the man page for `git submodule`
+
+>  Show the status of the submodules. This will print the SHA-1 of the currently checked out commit for each submodule, along with the submodule path and the output of git describe for the SHA-1.
+>  
+>  Each SHA-1 will possibly be prefixed with
+>  -  **-** if the submodule is not initialized
+>  - **+** if the currently checked out submodule commit does not match the SHA-1 found in the index of the containing repository 
+>  - U if the submodule has merge conflicts.
+
+The existence of the `+` would make this check fail. However, no prefixes will make the check succeed.
+
+By executing this function within the repositories `setup.py`, it verifies that the needed dependencies are checked out and clean before installing the python package. Checks like these make python packages feel a little more stable.
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